refactor(common): de-Qt oakcommon and wrap it in a pure C ABI

- de-Qt all classes under src/common (std::string/vector/mutex,
  std::filesystem, expat-based XmlStreamReader/Writer)
- add pure C ABI in include/common + src/common/c_api: opaque handles,
  init returns NULL on failure, free(NULL) is a no-op, out-params with
  negative OAKCOMMON_E_* error codes (include/common/error.h)
- remove single-consumer classes from oakcommon (html, jobtime,
  otioutils, playbackaudioclock, tohex, util, avframeptr,
  crashpadinterface/crashpadutils, autoscroll, digit, range); their
  destinations are recorded in docs/zh/plans/riir/notes.md
- add gtest suites under src/common/tests (127 cases), standalone
  build driver in src/common/standalone

src/common/c_api/CMakeLists.txt

@@ -0,0 +1,14 @@
+target_sources(oakcommon PRIVATE
+        commandlineparser.cpp
+        current.cpp
+        debug.cpp
+        dropworkflowbehavior.cpp
+        ffmpegutils.cpp
+        filefunctions.cpp
+        miscutils.cpp
+        ocioutils.cpp
+        oiioutils.cpp
+        power.cpp
+        qtutils.cpp
+        xmlutils.cpp
+)